The inductively coupled plasma - mass spectrometer (ICP-MS) is the analytical instrument of choice for trace element detection and quantification. Despite the popularity of ICP-MS, significant degradation in sensitivity and precision occurs as the result of matrix and instrument-induced effects. The sources of these effects are not well understood, characterized, or correlated to particular plasma operating condition settings or matrix compositions and involve both neutral and charged species. The purpose of this study is to characterize the behavior of metastable Ar (I) atom and Ca (II) ion through the measurement of Doppler velocities and fluorescence line width "temperatures." For the characterization of Ar (I), axial and radial velocity and temperature profiles were collected as a function of nebulizer rate, incident ICP power and matrix composition to establish a behavioral baseline for neutral species in the first vacuum stage expansion of an ICP-MS. Velocities were determined from the Doppler shift of laser-induced Ar (I) fluorescence compared to stationary population wavelength reference. Unambiguous evidence of a thick Mach disk forming 10-12 mm downstream and persisting through 17-18 mm downstream, under standard ICP conditions, conflicts with the widely held view of a thin Mach disk located between 15-17 mm downstream. Characterization of Ca (II) ion focused on the effect of changing ICP conditions and matrix composition on calcium ion Doppler velocity and temperature profiles in the first vacuum stage expansion. Evidence of the plasma potential acceleration of ions through the interface was found as a higher Ca (II) terminal velocity than that of Ar (I) under standard ICP conditions. Additionally, the effect of a lithium matrix on Ca (II) velocity and temperature profiles was generally opposite than on Ar (I) velocity and temperature profiles.